This invention relates to the field of petrochemical synthesis, i.e. to the technology of oxidation of cumene by an oxygen-containing gas (usually by air) to form cumene hydroperoxide (CHP) whose subsequent decomposition in the presence of an acid affords phenol and acetone. These reactions are the typical scheme for industrial heavy-tonnage production.
Two primary methods of producing cumene hydroperoxide (CHP) are known.
The first, so-called “dry” method is based on liquid phase oxidation of pure) cumene conducted in the presence of catalytic amounts of basic compounds, e.g.:                carbonates of alkali and alkaline-earth metals [See, e.g., U.S. Pat. No. 2,613,227 (1952), U.S. Pat. No. 2,619,509 (1952), U.S. Pat. No. 2,689,936 (1954)],        sodium bicarbonate [See, e.g., U.S. Pat. No. 2,577,768 (1951)],        calcium hydroxide [See, e.g., U.S. Pat. No. 2,632,774 (1953)],        barium oxide [See, e.g., U.S. Pat. No. 4,153,635 (1979)],        substituted ammonium salts [See, e.g., U.S. Pat. No. 4,192,952 (1980)], and other compounds that, in the process of oxidation, are suspended in cumene.        
The use of such basic compounds is advantageous for the following reasons. In the process of oxidation of cumene, trace amounts of organic acids, particularly formic acid, are formed along with the target product, CHP, and two main impurities, acetophenone (ACP) and dimethylphenylcarbinol (DMPC). The presence of formic acid in the cumene oxidation reaction mass inevitably leads to acid-catalytic decomposition of CHP with formation of phenol and acetone. It is known that phenol is a strong inhibitor of the free-radical oxidation of alkylaromatic hydrocarbons and cumene in particular. Thus, the presence of even trace amounts of formic acid significantly slows the oxidation process rate. Therefore, efforts are made to conduct the cumene oxidation reaction at pH˜5-7. The most obvious technique for removing acids from the cumene oxidation reaction mass is to conduct this process in the presence of the aforementioned advantageous basic compounds.
The second, so-called “wet,” aqueous-emulsion method of producing cumene hydroperoxide by oxidation of cumene consists of conducting the oxidation reaction in a three-phase system including:                an organic phase consisting of cumene and the products of its oxidation,        an aqueous phase consisting of solutions of basic compounds, and        a gaseous phase consisting of an oxygen-containing gas (usually air).        
Both the “dry” and the “wet” cumene oxidation methods are conducted in the presence of basic compounds.
Basic compounds dissolve in water much better than in hydrocarbons. Therefore, the mass transfer process in heterogeneous “organic phase—water” systems is much more effective than in “organic phase—solid dispersion” systems. So, in terms of the more complete and faster removal of acids from the system, the “wet” oxidation method should be recognized as more effective than the “dry” method.
This invention specifically relates to the “wet” method of cumene oxidation.
A method is known for producing CHP by oxidation of cumene by air at a high temperature. The oxidation reaction is conducted in the presence of ammonium salts of organic acids or carbonic acid; 0.05-50% aqueous solutions of the salts are used. The method (USSR Author's Certificate No. 567723, published on Sep. 9, 1977 in Bulletin of Inventions No. 29) has the following disadvantages, which are first discuss whit examples using ammonium salts of organic acids. Under high-temperature (80-120° C.) conditions of the cumene oxidation process, partial thermal decomposition of the salts occurs by the following reaction: where:                A is the symbol of an organic anion,        AH is the symbol of the acid of that organic anion.        
Since ammonia features a significant volatility, the liquid phase predominantly contains the acid that inhibits the cumene oxidation process. Moreover, it is economically inefficient to use ammonium salts of such relatively expensive organic acids as ethylenediaminetetraacetic or 1,10-decanedicarbonic acid.
If ammonium carbonate, an ammonium salt of carbonic acid, is used, then under high-temperature conditions, decomposition of the salt occurs according to the following reaction mechanisms: 
Reaction (1) predominately occurs in the aqueous phase while reaction (2) dominates in the organic phase.
An increase in the temperature shifts the equilibrium in Reaction (1) to the right while a decrease in the temperature shifts it to the left. Industrial cumene oxidation reactors are equipped with condensation systems whose function is to condense the carryover vapors of cumene and, partially, of water. In the condensation process that is conducted under lower temperature conditions, the equilibrium in Reaction (1), as mentioned above, shifts to the left, which results in a partial recovery of the alkaline agent in the cumene oxidation reactor. That circumstance has a positive effect on the process performance. On the other hand, Reaction (2) eventually leads to formation of carbamide (urea) whose aqueous solution has a much lower pH than the corresponding ammonia solutions. That circumstance inevitably leads to a worsening of the characteristics (rate and selectivity) of the oxidation process.
Furthermore, the salts are practically insoluble in organic phases while the volume of the organic phase represents the larger part of the solution. That is why the salts clog the pipelines and precipitate on the walls of heat-exchanging equipment, which leads to reduced heat transfer coefficients. This circumstance especially impairs the process of CHP rectification/concentration that follows the cumene oxidation step.
As follows from the description of invention (Author's Certificate No. 567723), that process is essentially a “dry” oxidation process since the amounts of the aqueous solutions added are so small (e.g., 50% solutions of ammonium carbonate are used in an amount of 0.17 g per 300 g of cumene) that all water is dissolved.
A process is known for producing cumene hydroperoxide using air oxygen in the presence of gaseous ammonia in the amount of no less than 0.5% of the reacted oxygen [U.S. Pat. No. 2,632,026 (1953)]. Although the cumene conversion (up to 21%) and CHP formation selectivity (up to 97.3%) characteristics of that process are good, its primary disadvantage is a very low oxidation rate.
The process has the following primary disadvantage: in feeding gaseous ammonia into the reactor, most of the ammonia escapes into the atmosphere. All existing CHP synthesis plants are equipped with waste-gas afterburning units (thermal afterburning units are used more often than the catalytic ones). This, in turn, leads to the presence of nitrogen oxides in the gaseous emissions and has a negative environmental impact. Furthermore, the patent's high conversion and selectivity characteristics are achieved at a very low cumene oxidation rate (0.6% cumene per hour). In its technical essence, the closest prototype of the proposed method is a process for producing cumene hydroperoxide by oxidation of cumene in an aqueous/alkaline emulsion at a temperature of 92-107.2° C. and a gage pressure of 5 atm in a horizontal cascade of reactors (no fewer than two) in two steps: cumene sequentially passes the first-step and second-step reactors into each of which the oxidant (air) is fed. In order to neutralize the acids, an aqueous solution of sodium carbonate is fed into the second step of the process; in the course of neutralization, sodium carbonate is transformed into sodium bicarbonate. The aqueous salt solution from the second step of the process is treated by ammonia or ammonium hydroxide up to pH=10.5-11.5; in the course of that process, sodium bicarbonate is transformed into the mixed salt, NH4NaCO3. The neutralized solution is fed into the first-step reactors in a ratio of (3.5-2.6):1 to the cumene that is fed for oxidation [U.S. Pat. No. 5,767,322, 1998: prototype].
U.S. Pat. No. 5,908,962, 1999, held by the same applicant, proposes, under the conditions similar to U.S. Pat. No. 5,767,322, feeding ammonia in an amount at least stoichiometric in relation to the amount of acids formed in the cumene oxidation process while monitoring the salts thus formed under ambient pH of 10.0-12.0 while ammonia is injected directly into the first-step reactors.
It is known that, in oxidation of cumene, two byproducts, dimethylphenylcarbinol (DMPC) and acetophenone (ACP), are formed along with CHP; the amount of those byproducts determines, ultimately, the yield of commercial products and the mass of the undesirable production waste, the phenolic resin; it also complicates the process of producing commercial products of the required quality. Therefore, an improvement of the cumene oxidation selectivity at a sufficiently high conversion (optimal CHP concentration in the flow leaving the oxidation unit is 25-30%) is an important issue for increasing the effectiveness of the industrial technology.
The following disadvantages of the prototype process can also be indicated:                using different neutralizing solutions for the first and second oxidation steps, which complicates the technological scheme;        the presence of sodium salts, which can precipitate on the walls of heat-exchanging equipment;        leads to reduced heat transfer coefficients;        furthermore, the large amount of the neutralizing aqueous solution in relation to the cumene that is being oxidized, (3.5-2.6):1. Providing for a required capacity of the oxidation plant results in larger reactor volumes compared to the “dry” oxidation method.        